Asymptomatic Covid-19 Cases Show Need for Wider Surveillance Testing, Study Suggests

study of Covid-19 testing among nearly 2,000 young adults found symptom monitoring missed nearly all cases of infection, suggesting regular, widespread surveillance testing is needed for both asymptomatic and symptomatic people to get the coronavirus crisis under control.

The research, published in the New England Journal of Medicine on Wednesday, was among the largest to look at asymptomatic transmission of the coronavirus. The study looked at 1,848 U.S. Marine Corps recruits between the ages of 18 and 31, who had been required to quarantine at home before arriving at the Citadel military college in Charleston, S.C., where they underwent a second 14-day on-campus supervised quarantine before beginning training.

At Citadel, they were monitored every day for symptoms and were scheduled to be tested three times—once within the first two days of arrival, then on day seven and again on day 14. By the 14th day, 51 of the study participants had tested positive for the coronavirus.

What was surprising to researchers: All 51 cases of Covid-19 were picked up by the prescheduled tests. None was detected as a result of additional tests given to individuals who reported symptoms.

The findings come as coronavirus infections are surging, with the U.S. recently reporting a record 136,000 new Covid-19 cases in a single day. Daily caseloads have hit record levels in several states, and hospitalizations are at their highest level since the pandemic began. The total confirmed Covid-19 case tally in the U.S. has now surpassed 10.3 million.

Only five people of the 51 who tested positive reported having any symptoms, according to the study. Their symptoms likely didn’t meet the threshold for getting referred for an additional test, researchers said.

Although recruits slept two to a room, used shared bathroom facilities and ate in shared dining facilities, they were required to wear double-layered cloth masks at all times, indoors and outdoors, except when eating or sleeping. They also practiced social distancing at least 6 feet from others and weren’t allowed to leave campus. Six supervisors were assigned to each platoon, working in eight-hour shifts to enforce the quarantine measures.

Despite the stringent measures and initial testing, the virus still slipped through the cracks and spread among recruits, the researchers said. They ran genetic analyses on some of the infected volunteers’ samples and found six clusters of volunteers where the viral genomes shared a high degree of similarity. The people with similar-looking virus genomes likely transmitted the virus to each other, the researchers said.

“This shows that even in the setting of very strictly enforced public-health measures put in place to mitigate spread, if you want to find the individuals who are infected with SARS-CoV-2, you really have to augment those public-health mitigation strategies with additional surveillance testing,” said Andrew Letizia, one of the lead authors of the study and an infectious-disease expert at the Naval Medical Research Center.

Public-health experts describe surveillance testing as testing people on a regular basis, regardless of whether they have symptoms, to identify infected people as soon as possible, before they can spread Covid-19 to others. That can be done by testing people one by one, or through what is known as pooled testing, where several patient samples can be screened using a single test. Many people, especially young people, don’t have symptoms when infected, the experts say.

For the general population, the Centers for Disease Control and Prevention recommends testing for people who have symptoms of Covid-19, as well as asymptomatic people who have had close contact with someone with confirmed Covid-19 or have been referred by their health-care provider or local or state health department. In some settings, such as workplaces where physical distancing is difficult and schools where the transmission risk is moderate to high, the agency says that broader, periodic testing of people without symptoms can be useful in detecting Covid-19 early and stopping transmission quickly.

“There are literally invisible outbreaks happening all over the place until those outbreaks hit up against vulnerable people,” said Michael Mina, assistant professor of epidemiology at the Harvard T.H. Chan School of Public Health, who wasn’t involved in the study.

Many places still rely on symptom-based Covid-19 tests to flag infections, but the study shows they won’t be enough to stop the coronavirus spread, especially as cases continue to rise, said Stuart Sealfon, a professor of neurology at the Icahn School of Medicine at Mount Sinai and one of the lead authors of the study.

“In this population of recruits, [symptom-based testing] was completely unable to find a case,” Dr. Sealfon said. “This very strongly indicates that extensive and repeated surveillance testing is going to be a critical component in suppressing infection in this population and in general.”

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